The representation of images which have a three-dimensional effect is based on the reproduction of two partial images on a picture screen using stereo projection. A first partial image reproduces a scene in the way that the left eye of a viewer would see it, while a second partial image reproduces it in the way that it would appear to the right eye of the viewer. In that situation, the viewer looks through special spectacles which provide each eye only with the image which is intended for same. By virtue of separate perception of the partial images with the left eye and the right eye, the viewer has the impression of three-dimensional perception.
This effect can be produced by means of active or passive stereo projection methods. Active methods project partial images for the left eye and the right eye alternately at high frequency (about 100 Hz). The viewer wears a pair of filter spectacles whose lenses have a variable degree of transmission. The partial image which is not wanted for the respective eye is blocked out by the light transmission of the lens in question being switched synchronously to a low value for image change purposes. That can be controlled for example by means of infra-red signals by the image computation and output system. Active methods afford the advantage that basically only one projector is required for each projection surface, which projector alternately projects partial images for the left eye and the right eye.
Passive methods project the partial images for both eyes at the same time, but involving different polarization of the light. They therefore require two projectors for each projection surface, the partial images of which are always simultaneously projected onto the projection surface where they are superimposed on each other. The partial image for the left eye can be for example polarized horizontally and the partial image for the right eye vertically. Left-circular and right-circular polarization is also used. The viewer again wears a pair of filter spectacles whose lenses respectively only transmit light of one polarization direction. In the first-mentioned example the left spectacle lens would only transmit horizontally polarized light and the right spectacle lens would only transmit vertically polarized light. The partial image which is unwanted for the respective eye is blocked by virtue of its “wrong” polarization. Passive stereo projection methods can operate at lower refresh frequencies than active methods as each projector serves only one eye. For example frequencies of about 50 Hz are used.
Various arrangements with different numbers of projection surfaces are known. If two or more projection surfaces are used they are generally perpendicular to each other in order to cover a field of vision of the viewer which is as large as possible, and thereby to give him a more realistic three-dimensional impression. Two partial images are formed on each projection surface, of which partial images each respective one reproduces a part of the field of vision of an eye. If an image which has a three-dimensional effect is produced not only on four mutually perpendicularly disposed walls but also on floor and ceiling, the three-dimensional optical impression is perfect and the viewer thinks that he is “immersed” in the illustrated scene.
Polarization-maintaining scattering walls are used for image projection in passive methods. Polarization-maintaining scattering walls are commercially available both in the form of back-scattering and also light-transmitting, forward-scattering arrangements. Image projection onto the scattering wall can therefore also be effected when using passive methods from the side of the viewer or from the side of the scattering wall, which is remote from the viewer.
The use of linearly polarized light involving mutually perpendicular vibration directions of the electrical field vector has the advantage in regard to the passive methods that the suppression of the respective other polarization direction by the spectacle lenses is particularly strong. However, the proportion of “wrongly” polarized light which is transmitted by both spectacle lenses rises when the viewer inclines his head. For that reason, arrangements which also project an image having a three-dimensional effect onto the floor and/or at the ceiling basically use circularly polarized light. The disadvantage of that method is the poorer suppression of the respective “wrongly” polarized light.
In the meantime, interactive projection systems for three-dimensionally acting images are known, in which the viewer can alter the illustrated scene, by means of an input device. For example, the viewer can alter his apparent viewpoint or angle of view relative to the illustrated three-dimensional scene by means of suitable control commands and thus has the impression of moving within that scene as in an actual three-dimensional environment.
Besides one or more projection surfaces and a corresponding number of projectors, interactive systems of that kind have one or more input devices. The input devices used for example are sensors which are fixed to the head of the viewer and which make it possible to determine the direction of view, as well as pointing devices or sensor gloves. The input device and the projectors are connected to a graphics computer which, on the basis of the signals of the input device for each projector, computes the next scene which is to be illustrated and outputs it thereto.
A particularly important aspect of projection control is synchronization of the partial image projection on the different projection surfaces. To achieve the three-dimensional effect, the various partial images for the left eye and for the right eye must be respectively simultaneously visible on all projection surfaces. This means that all partial images must simultaneously represent the overall scene in one and the same phase (the same “frame”). Image computation (“rendering”) for the individual partial images may therefore not “diverge” in such a way that one partial image illustrates the scene at a more advanced stage than another.
This problem is resolved in an active stereo projection system known by the trademark “CAVE” having six projection surfaces by means of an integrated computation and control system for the partial images to be projected of all six projectors. For that purpose this system uses an SGI Onyx2-computer from Silicon Graphics with twelve R10000 processors, a 4 Gigabyte working memory and a graphics unit with three InfiniteReality Graphics Pipes, each of which includes two raster managers with 64 Megabyte working memory.
The disadvantage of that system is that the computation and control system used is a highly developed expensive special item which is cost-intensive in terms of procurement and maintenance.